Despite grids' popularity, virtual organizations (VOs) have yet to become a commodity technology in modern computing environments due to the complexity of managing them and difficulty of assuring user and VO isolation. Here, the authors describe the VO management approach taken by XtreemOS, a new grid operating system with native support for VOs that supports a wide range of computing resources, from clusters to mobiles. They also discuss the requirements for the VO model and management within XtreemOS and intro duce an expandable VO model and a system architecture that supports it.